Right now I’m in the midst of incorporating this feature into the release plan. The GData APIs for cocoa are making this straight forward enough, but the tricky part is making the experience fit in with the overall Mac user expectation.
Once you can authenticate to either site, you need to present the complex behavior of the web application in a simplified manner. Should users be able to only upload? Should they be able to view previously uploaded images (to ensure they’re not duplicating things)? Should they be able to edit tags and descriptions for images already uploaded, or just when they initially upload them? Complexity grows quickly.